1. Field of the Invention
This invention generally relates to a light emitting device (LED) and, more particularly, to a three-terminal LED with built-in electrostatic discharge (ESD) protection for LED array applications.
2. Description of the Related Art
FIG. 1 is a schematic diagram depicting an equivalent circuit for a matrix type light emitting diode (LED), which is also known as a light emitting device (prior art). The luminous efficiency of LEDs has been enhanced drastically in recent years, and it is expected that the LED will replace the conventional incandescent bulb and fluorescent lamp in the future. To generate enough light from an LED to replace an incandescent bulb or fluorescent lamp for general lighting purpose, the area of the LED must be sufficiently large. However, the uniformity of light generated by an LED deteriorates as the LED area size increases. In addition, the heat generated by a large LED is concentrated, and has an adverse effect of LED performance. Therefore, it is a common practice to fabricate small LEDs and electrically connect them to form a matrix or an array as shown in the figure. The diode symbol in the figure represents a light emitting device or LED diode. The matrix can have M columns and N rows, where M≧1, N≧1, and M×N≧2. The integration of the LED matrix can be monolithic, meaning that the LEDs are fabricated on a common substrate and electrically connected on the same substrate. Alternatively, the LED matrix can be formed at the packaging level, meaning that LED diodes are dicing from a growth substrate, LED dies are mounted on a package, and the electrical connections are made on the package. No matter what kind of connection is made, the final LED matrix has two external pads for electrical connection.
FIG. 2 is a schematic drawing depicting the LED array of FIG. 1 with a two-way Zener diode to protect the LEDs during an electrostatic discharge (ESD) event (prior art). For lighting applications, group III nitride semiconductors (III-nitride) have attracted much attention because the bandgap energy of AlxGa1-xInyN varies from 0.78 electron volts (eV) of InN to 6.3 eV of AlN. Thus, the emitting wavelength of III-nitride LEDs can be easily adjusted by changing the composition ratio among aluminum (Al), gallium (Ga), and indium (In). Although GaN based LEDs are commercially available, ESD is still a problem due to the fact that the GaN is usually grown on an insulating substrate, such as sapphire or SiC. When an ESD event comes to contact with the LED pad, a surge voltage can destroy the device instantly. To protect the LED matrix, an externally connected two-way Si Zener diode may be placed between two pads as shown in the figure.
FIGS. 3A and 3B are schematic diagrams depicting a protection diode that is reverse and parallelly connected to an LED (prior art). In FIG. 3A each LED diode has one protection diode. In FIG. 3B each row of LEDs has one protection diode. When an ESD event comes to contact with the LED pad, and the LED diode is forward biased, the LED diode discharges the ESD current. If the ESD event comes to contact with the LED pad, and the LED is reverse biased, then the parallel-connected ESD protection diode is forward biased and discharges the ESD charge. In additional to protection from an ESD event, the protection diode can protect the array from degradation due to the “partial shading effect” described below. An LED is a PN diode that is fabricated on a semiconductor material, and it typically operates in the forward bias condition. If light illuminates an LED while it is off, the LED behaves like a solar cell, which can convert the photo energy to electrical energy. If an LED is in a series-connected string that is shaded, with the remaining LEDs still under the light, the shaded LED is forced into reverse bias breakdown so that it can conduct the current of the string. The reverse biasing of the LED can permanently damage the cell. The two-way Si Zener diode in FIG. 2 cannot protect the LED from damage due to this effect. This effect is commonly seen in the solar cell industry. To prevent this type of stressing on a shaded solar cell, the common solution is to reverse and parallelly connect a discrete diode. A reverse and parallelly connected discrete diode protects an LED from ESD damage, as well as the damage due to the voltage build up on a shaded LED when other LEDs are under illumination.
FIGS. 4A and 4B are, respectively, partial cross-sectional and schematic views LED and ESD diodes are fabricated on the same substrate. The PN diodes are reverse and parallelly connected together. The protection diode in FIG. 3A or 3B can be a diode made of different semiconductor material than the LED diode. For example, the protection diode can be Si diode and the LED can be GaN LED on sapphire substrate. The LED dies and the protection diodes can be package together. Although the Si diode is cheap comparing to the LED diode, the packaging of the two together can be costly. To reduce the cost of the LED matrix, the protection diode and the LED can be made of same semiconductor materials and can be fabricated together. It is well known to monolithically integrate LEDs with a reverse and parallelly connected ESD diode. The ESD diode can be a PN diode or a Schottky diode.
Shown is a GaN LED (150) that is protected with an ESD diode (160), which is fabricated on the same insulating substrate (101). 103 is an n-type GaN-based clad layer; 105 is an active layer; 107 is a p-type GaN-based clad layer; 109 is a transparent electrode; and 110, 112, 114, and 116 are metal electrodes. 110 and 114 are connected by 120, and 112 and 116 are connected by 130. An isolation etch at 140 ensures isolation between LED and ESD diodes. The film thicknesses are not drawn to scale, with GaN layer 103 being much thicker than the overlying films. The disadvantage of using this approach is that a deep GaN etch is needed to ensure the isolation between the LED and ESD diodes. In addition, the metal deposition process must typically be performed in two steps.
It would be advantageous if a monolithic chip or die could be made that included both an LED and protection diode, without the requirement of a deep isolation etch through the n-doped semiconductor substrate.